The present invention generally pertains to presence-detection-system tags that include transponders and is particularly directed to improvements in tags that include frequency-dividing transponders of the type that includes an active strip of magnetic material that frequency divides when in the presence of a magnetic bias field of a predetermined intensity.
This type of frequency-dividing transponder is described in U.S. Pat. No. 4,727,360 to Lucian G. Ferguson and Lincoln H. Charlot, Jr., which is assigned to the assignee of the present application. The transponder described therein is contained in a presence-detection-system tag with a bias strip of magnetic material that, when magnetized, biases the active strip of magnetic material to be within the predetermined magnetic field intensity range so as to enable frequency division by the transponder. The frequency-dividing transponder is deactivated by demagnetizing the bias strip of magnetic material. In a typical prior art tag of this type, the bias strip of magnetic material is a strip of 1095 carbon steel of specific dimensions that has been magnetized along its length to create opposing magnetic poles at each end, and having a coercivity (H.sub.C) in a range of from 20 to 40 Oersteds. This coercivity range is low enough to allow for deactivation of the transponder by applying a transverse stroke to the bias strip with a compact high strength magnet to thereby alter the magnet pole arrangement of the bias strip.
Use of a high-coercivity magnetic material in the bias strips would make the tag more resistant to deactivation.
An extruded high-coercivity magnetic material is less expensive than ceramic and metal alloy magnets, such as the Alnico series, which often require grinding and machining, and is also less expensive than other alternatives, such as injection molded, pressed or plastiformed magnets.
Extruded high-coercivity material is produced in long ribbons having a maximum width of approximately fifty millimeters and a magnetic moment parallel to the lateral dimension of the ribbon. However, in order to meet preferred design parameters, the active strip of magnetic material in the transponder tag may be of a given length that is greater than fifty millimeters, whereby if the length of the bias strip of magnetic material is no more than 50 millimeters in length, the bias field is distorted at the ends of the active strip and the transponder does not perform satisfactory.